I used the insulation board that so many others have mentioned. I looked online and found 2" grid flannel made by Mayfair, I think. I cut the boards to the right height, covered the boards with the grid flannel, taped it to the back with packing tape, then mounted to the wall with screws in the corners. I covered each board separately since the flannel was only that wide. Works great to keep my blocks lined up straight.